Prof Mario Clerici, from the University of Milan, told the BBC News website: "Just a couple of hours after infection, the virus has already started seeding the organs and hides so therapy cannot eradicate HIV.

"You can treat patients, but you cannot cure them. Right now it is impossible."

In July, a baby girl in the US born with HIV and believed cured after very early treatment was found to still harbour the virus.

Doctors said tests on the four-year-old child from Mississippi indicated she was no longer in remission.

She had appeared free of HIV as recently as March, without receiving treatment for nearly two years.

"A cure for HIV is still at ground zero," said Prof Clerici.

Distant prospect

Commenting on the findings Prof Sanjaya Senanayake, from the Australian National University Medical School, said: "This case shows that undetectable HIV in the blood does not mean that the body is free from virus and that there is still some way to go before a cure is found."

Only one person has been "cured" of HIV.

In 2007, Timothy Ray Brown received a bone marrow transplant from a donor with a rare genetic mutation that resists HIV.